improve waitUntilSettled logic

1) introduce a timeout so the process shuts down even tho it hasn't send all messages to the parent
2) don't make sentMessages bump dependent on whether or not the send method returned with true or false, Mocha sometimes sends to many events at once so the buffer "exceeds a threshold that makes it unwise to send more" (https://nodejs.org/api/child_process.html#child_process_child_send_message_sendhandle_options_callback). These messages didn't fail to get sent (see nodejs/node#7657 (comment))

lib/adapter.js

@@ -22,6 +22,7 @@ const EVENTS = {
 }
 
 const NOOP = function () {}
+const SETTLE_TIMEOUT = 5000
 
 /**
  * Mocha runner
@@ -234,9 +235,8 @@ class MochaAdapter {
             this.sendInternal(event, message)
         }
 
-        if (this.send(message, null, {}, () => ++this.receivedMessages)) {
-            this.sentMessages++
-        }
+        this.send(message, null, {}, () => ++this.receivedMessages)
+        this.sentMessages++
     }
 
     generateUID (message) {
@@ -314,8 +314,11 @@ class MochaAdapter {
      */
     waitUntilSettled () {
         return new Promise((resolve) => {
+            const start = (new Date()).getTime()
             const interval = setInterval(() => {
-                if (this.sentMessages !== this.receivedMessages) return
+                const now = (new Date()).getTime()
+
+                if (this.sentMessages !== this.receivedMessages && now - start < SETTLE_TIMEOUT) return
                 clearInterval(interval)
                 resolve()
             }, 100)